Two general approaches to the study of smooth muscle function are proposed. I. Studies into the mechanism of oxygen sensitivity of the ductus arteriosus of the newborn guinea pig are proposed in order to obtain insight into the relationship between oxidative metabolism and contractile activity. Efforts will center on defining the electrical changes which underly the response to oxygen and the relationship of intracellular high-energy phosphate levels and contractile activity. II. A single smooth muscle fiber preparation will be utilized in order to obtain insight into the arrangement of the contractile apparatus and the relation of electrical and mechanical activity. Large single smooth muscle cells will be obtained from the stomach muscularis of two amphibian species, Bufo marinus and Amphiuma means. A means for attaching levers to a cell wll be devised. A length-tension analysis will be perfomed on the single cell and used to evaluate various models for the arrangement of contractile elements within the smooth muscle cell. Surface membrane labeling techniques will be utilized to investigate the distribution and lability of attachments between the contractile apparatus and the surface membrane. The ionic basis for electrical activity in smooth muscle will be studied using electrophysiological techniques on individual cells and ion flux studies on cell suspensions. The role of cyclic nucleotides in the contractile response to excitatory and inhibitory agents will be studied by measurement of their levels in suspensions of isolated smooth muscle cells during the response to such agents.